Abstract
Microalgae are a promising source of high-value bioactive compounds, which are enclosed by rigid cell walls. Algal cell disruption is critical to allow release of the biofunctional compounds. High pressure homogenisation (HPH) is a highly efficient cell disruption method that has structure-enabling potential. There is currently a gap of information regarding the impact of HPH as a structure-enabling process in influencing the different physicochemical and functional properties of microalgae with potential as food ingredients.
This PhD thesis aimed at investigating the impact of HPH on microstructure modification and the rheological, volatile, and health-promoting properties of microalgal suspensions. Four microalgal species, namely, Arthrospira sp., Isochrysis sp., Nannochloropsis sp., and Tetraselmis sp. were characterised and the microalgal suspensions were subjected to HPH treatment at different homogenising pressures (300, 600, 900 bar). Each of the microalgal species were characterized to have distinct compositional, rheological, and volatile characteristics.
HPH led to severe disruption of Arthrospira sp., Isochrysis sp. and Tetraselmis sp. cells in the microalgal suspensions with Arthrospira sp. exhibiting cell fragmentation whereas both Isochrysis sp. and Tetraselmis sp. exhibited particle aggregation and flocculation. The microalgal suspensions showed a general trend of increasing viscosity at elevated homogenising pressure. Arthrospira sp. suspensions exhibited the most pronounced effect of HPH and had the highest thickening potential that could be attributed to the high protein content. In contrast, the rigid cell walled-Nannochloropsis sp. were resistant to shear damage by HPH with minimal impact on cell disruption and rheological properties.
HPH had minimal impact on the measured fatty acid profile of all microalgal suspensions, which generally exhibited abundance of nutritionally relevant polyunsaturated fatty acids (PUFA). HPH had a substantial impact on the volatile composition of the microalgal suspensions. Advanced chemometric approach allowed the identification of discriminant compounds that provided insight into the chemical reactions that were possibly triggered by HPH. Lipid oxidation reactions were observed in the HPH-treated suspensions of all four microalgal species as indicated by changes in the amounts of hydrocarbons, alcohols, and aldehydes at increasing homogenising pressure level. Carotenoid degradation may have occurred during HPH as indicated by terpenes and ketones in Arthrospira sp., Isochrysis sp., and Nannochloropsis sp. suspensions. HPH-induced Maillard reaction was associated with the detection of pyrazines and furans in processed Arthrospira sp. suspensions. Additionally, HPH may influence the degeneration of sulphur-containing amino acid, which was associated with the altered concentration of dimethyl sulphide in treated Tetraselmis sp. suspensions.
HPH had generally minimal and no clear effect on the in vitro protein digestibility of the microalgal suspensions. Increasing homogenising pressure led to higher measured pigments in the gastrointestinal digests of the Arthrospira sp. and Nannochloropsis sp. suspensions. Nannochloropsis sp. gastrointestinal digests were positively impacted by HPH with the increased phenolic content and antioxidant activity at elevated homogenising pressure. Arthrospira sp. gastrointestinal digests treated at 600 and 900 bar exhibited bioprotective effect on oxidative-stressed Caco-2 cells.
This PhD thesis increased our understanding into the potential of HPH as a cell disruption method to influence the microstructural, rheological, volatile, and health-promoting properties of microalgal suspensions. The effect of HPH on these functional properties were deemed to be species- and pressure dependent. The structuring capability of microalgae as a functional ingredient can be exploited with optimal HPH processing conditions to allow more diverse applications in the food and pharmaceutical industries.